1. Field of the Invention
The present invention relates to mobile communications and more particularly to an apparatus for judging the quality of data communication between a fixed station and various mobile units including automobiles and humans or data communication among mobile units.
2. Description of the Prior Art
Recent rapid progress of social and economical activities and diversifications thereof increasingly require telecommunication service to be qualitatively expanded as well as to be quantitatively improved.
Especially, such mobile communication plays an important role as a means capable of effecting communication at any time and from any place.
Mobile communication of this type has the following features in comparison with other communication means:
(1) Radio means is indispensable to mobile communications. PA0 (2) Mobile communications can be established by combining a radio system, an exchange system, terminal equipment, parts technologies, etc. PA0 (3) Applicable radio frequencies are limited. PA0 (4) Radio-wave propagation is complicated.
Although in the mobile communication system a cell site is fixed, a mobile unit gets about. Accordingly, they do not necessarily afford unobstructed views of each other.
In particular, the visibility therebetween is sometimes bad along roads in an urban area due to buildings lying therebetween.
Moreover, a mobile station employs a non-directional antenna to make it possible to communicate with a cell site irrespective of its direction.
Accordingly, any radio-wave propagating between a mobile station and a cell site through spaces among many buildings in an urban district is complicated owing to the influences of reflected waves from surrounding building, so that traveling of a mobile unit causes the strength of a received electric wave to be widely varied.
The period of the variations of the radio-wave is associated with a travelling speed of a mobile unit as well as the radio frequencies used. With the faster traveling speed and with the higher frequency of the radio-wave used, a fading phenomenon with a more rapid period is produced.
For example, with a traveling speed of 40Km/H over a 800 MHz band, a received electric field is varied in time about thirty times in one second.
It is known from many efforts to investigate such radio-wave propagation that the strength of a received electric field in an urban area becomes lower as a mobile is further away from a cell site, and becomes a considerably lower value than a theoretical value on a radio-wave propagating along the plane ground having a good line of sight.
In effecting data communication under such a poor environment, a received electric wave was simply displayed on a display in an analog or digital manner up to now.
This raises no trouble in the case of a man-to-man communication since a user properly may judge the quality of conversational communication.
However, in applying the system to simply display a received signal or to display data communication, judgment by a user, even if suitable, may cause frequent digital code errors, and makes it difficult to satisfactorily establish data communication and assure good quality data communication.
Accordingly, many efforts to devise an apparatus for judging the quality of mobile data communications have been desired at present.